Voltage regulators (VRs) and DC-to-DC converters are widely used for providing electrical power for computer processors and telecommunications electronics. Many voltage regulators include circuits for measuring output current and voltage so that feedback control of the voltage regulator is possible. One method for feedback control is adaptive voltage positioning (AVP), in which the output voltage is controlled in response to the output current. AVP is particularly well suited for use in microprocessor voltage regulators and voltage regulators for memory and graphics-processing circuits. AVP typically requires accurate and high speed sensing of output current.
One method for accurate and high speed output current sensing is known as direct current resistance (DCR) sensing. DCR sensing is described in U.S. Pat. No. 5,982,160. FIG. 1 illustrates a buck regulator circuit employing DCR current sensing. Top and bottom switches Q1 Q2 are alternately switched and provide current pulses to output inductor Lo. Output filtering capacitor Co is connected in parallel with the load, as well known in the art. Output inductor Lo has an ideal, zero-resistance self-inductance L in series with a DC (ohmic) resistance RL. The DCR current sensing method requires a capacitor C and resistor Rc connected in parallel with the output inductor Lo. Voltage across the capacitor C is provided to a voltage buffer B. The capacitor C and resistor Rc are selected so that the RC time constant matches the L/RL time constant of the output inductor Lo. With the capacitor C and resistor Rc selected in this way, the voltage across the capacitor C is accurately proportional to the voltage across the DC resistance RL of the inductor, and, hence, the current flowing through the output inductor Lo. The voltage buffer B produces a voltage (the “current sense voltage”) that is proportional to the current flowing through the output inductor Lo. The output of the voltage buffer B is therefore accurately proportional to the current flowing through the output inductor Lo. The current sense voltage can be used for overload protection control, AVP or other feedback-based methods for controlling the operation of the buck regulator. A significant advantage of the DCR sensing technique is that it does not require a current sensing resistor, which dissipates energy. Hence, DCR sensing significantly improves the energy efficiency of the VR circuit.
The buck regulator of FIG. 1 has a single output inductor Lo. It is relatively simple to apply the DCR output current sensing technique to a voltage regulator circuit having a single output inductor. However, many new voltage regulator circuits, such as multi-phase buck regulators, and other more complicated resonant or quasi-resonant circuits, have coupled output inductors. Coupled output inductors tend to allow a reduction in the capacitance of output capacitor Co and therefore a reduction in cost. Also, coupled output inductors tend to improve transient performance and efficiency. For these reasons, coupled output inductors are increasingly used.
In the art, there is no known method for applying the DCR current sensing technique to coupled inductors. DCR current sensing is presently incompatible with coupled inductors. If a DCR current sensing circuit is connected to a coupled inductor, the capacitor voltage waveform will have a different shape than the inductor current waveform. Consequently, it is difficult to implement AVP (and other forms of feedback control that require accurate current sensing) in VRs that have coupled output inductors. Hence, circuit designers presently must choose between the benefits of DCR or the benefits of coupled output inductors.
It would be an advance in the arts of voltage regulators, current sensing circuits, and feedback control for VRs to provide a DCR current sensing technique compatible with coupled inductors. A DCR current sensing technique compatible with coupled inductors could extend the benefits of DCR current sensing to multiphase buck regulators and other regulators having coupled output inductors.